Preparation of hydro-thermal surface-densified plywood inspired by the stiffness difference in “sandwich structure” of wood

Inspired by stiffness difference in the “sandwich structure” of earlywood and latewood, surface-densified (SD) plywood was manufactured based on a hydro-thermal approach. A series of veneer moisture contents (MCs) and hot-pressing temperatures were selected to prepare the SD plywood. Great surface densification was observed according to density distribution through plywood thickness. The densification was attributed to the thermal softening and mechano-sorptive behavior of constituents in the wood cell wall. A greater degree of surface densification could be achieved at higher hot-pressing temperatures or higher surface veneer MCs. Final MCs of the SD plywood met the requirement of the Chinese national standard. No significant variation was observed for the bonding strength among SD and control (CK) plywood. Mechanical properties - including modulus of rupture (MOR), modulus of elasticity (MOE) and hardness - of the SD plywood were significantly higher than the CK plywood. The enhancements of MOR, MOE and hardness could be attributed to its high-density surfaces. The greatest increment of MOR, MOE and hardness was 54, 104 and 144%, respectively. Comparing to some domestic Chinese hardwood species, SD plywood possessed greater surface hardness with less average density. The results indicated that, inspired by the stiffness difference in the “sandwich structure” of wood, preparation of the SD plywood based upon hydro-thermal treatment is feasible.